Piston pumps are well known in which a piston-forming element is reciprocally slidable coaxially within a piston chamber-forming member as for dispensing fluids from a reservoir. One typical such piston pump is described in U.S. Pat. No. 5,975,360 to Ophardt, issued Nov. 2, 1999, the disclosure of which is incorporated herein. In such pumps, sealing engagement between sealing surfaces of the piston and sealing surfaces in the chamber are required to provide advantageous pumping. The present inventor has appreciated that difficulties arise in such piston pumps when the piston forming element and the piston chamber may adopt positions in which they are not coaxially aligned. Such performance issues in severe cases can include loss of dosage, failure to prime and leakage. In such prior art pumps as taught by the above-noted patent, the failure of the piston-forming element to be properly maintained coaxially within the cylinder can result in discs carried by the piston be moved into positions in which they do not form an adequate seal with a cylindrical wall of the piston chamber.